CHUJRNING AND WASHING BUTTER 251 
sterile. It used to be thought by older German investigators 
that these different filtering substances had almost miraculous 
power of removing organisms from water. 
The factors which are to be considered in successful filtra- 
tion are: 
(1) Storage capacity for unfiltered water. 
(2) Construction of filter-beds. 
(3) Rate of filtration. 
(4) Renewal of filter-beds. 
(1) Concerning the storage capacity, nearly all creameries 
have storage-tanks overhead in the creamery; so far as that 
is concerned, however, filtration can be successfully carried on 
continuously as well as intermittently. 
(2) The construction of the filter-bed used in the experi- 
ment carried on at the Iowa Experiment Station, Ames, Iowa, 
is as shown in Fig. 153. The approximate proportionate 
depth of each layer in the bed is as follows, beginning at 
the bottom: 
Two inches small flint stones; 22 inches fine sand; 12 
inches fine coke; 9 inches charcoal; 2 inches fine stone or coarse 
gravel. The layer of fine sand should not be less than 15 inches. 
It has been asserted that a few pieces of old iron mixed in 
the filter-bed are beneficial. Alum, lime, and copperas have 
been recommended for clarifying and deodorizing very impure 
water. As these substances are soluble they should not be used 
in filter-beds, which are intended for the filtration of water 
for potable purposes. The filtering-can was made from 22 
galvanized iron. The height of can is 48 inches; diameter, 
18 inches. The bottom of the can is slanting towards the 
faucet, or opening. Thus no water is permitted to stand on 
the bottom and afford opportunities for germs to accumulate. 
On the inside are three plates. One lies horizontally, near the 
bottom, and upon it the filtering-material rests. Another lies 
on the top of the fine sand. Both of these plates were per- 
forated with many small holes. Near the top is placed a 
concave plate with a hole near the center. This plate directs 
